A typical internal combustion engine for a vehicle includes a variable valve timing mechanism for changing the valve timing of intake valves and exhaust valves based on the engine operating state to improve the engine output and the exhaust characteristics. The mechanism changes the valve timing of the intake valves and the exhaust valves by changing the relative rotational angle of a camshaft with respect to the crankshaft of the internal combustion engine.
An variable valve duration mechanism has been proposed that changes the valve duration of the intake valves (the crank angle from when each intake valve is opened until when the intake valve is closed) in accordance with the engine operating state (for example, Japanese Laid-Open Patent Publication No. 2001-263015). With the internal combustion engine equipped with the mechanism, the amount of air drawn into engine combustion chambers is adjusted by changing the valve duration of the intake valves. Furthermore, adjusting the intake air amount by changing the valve duration of the intake valves permits the opening degree of an intake throttle valve to be set as large as possible, thus reducing the pumping loss.
Since the valve timing of the intake/exhaust valves and the valve duration of the intake valves are controlled in accordance with the engine operating state, the valve overlap amount of the intake/exhaust valves is appropriately set in accordance with the engine operating state. Since the internal EGR amount, the pumping loss, or the engine combustion state changes significantly as the valve overlap amount changes, the valve overlap amount is desirably controlled appropriately when operating the engine.
There is a predetermined delay in the response of the operation of the variable valve timing mechanism and the variable valve duration mechanism. Therefore, a temporary divergence occurs between the control target values and the actual values of the mechanisms. Therefore, when the control target values are changed in accordance with the change of the engine operating state during a steady operating state where the control target values and the actual values of the mechanisms coincide each other, the operating state of the internal combustion engine shifts into a transition state where the control target values and the actual values of the mechanisms diverge. Then, the operating state of the internal combustion engine shifts into the steady operating state again after a predetermined period has elapsed. When the valve timing of the intake/exhaust valves and the valve duration of the intake valves are independently controlled, the following problems occur when the operating state of the engine is in transition.
That is, when the internal combustion engine shifts from the steady operating state to the transition operating state, the valve timing of the intake/exhaust valves and the valve duration of the intake valves are controlled such that the control target values coincide with the actual values. However, at this time, the valve overlap amount may be controlled to an amount inappropriate for the engine operating state in transition. More specifically, as shown in FIGS. 7A to 7C, when, for example, the engine operating state shifts from a state where the valve overlap amount is desirably set as small as possible (FIG. 7A) to a different state (FIG. 7C), if the valve timing of the intake/exhaust valves and the valve duration of the intake valves are controlled accordingly, the valve overlap amount may be temporarily increased (FIG. 7B). When the valve overlap amount is unexpectedly increased, an internal EGR is excessively performed, leading to deterioration of the engine combustion. As described above, when the operating state of the engine is in transition, the valve overlap amount may be controlled to an amount inappropriate for the engine operating state on each occasion. Therefore, although it is temporary, there is an adverse effect on the engine operation.
To avoid adversely affecting the engine operation, for example, the valve timing of the intake/exhaust valves and the valve duration of the intake valves may be controlled such that the valve overlap amount will not be inappropriate when the operating state of the engine is in transition. However, when such a method is simply employed, although the valve overlap amount is appropriately controlled, the divergence between the actual values and the control target values of the valve timing of the intake/exhaust valves and the valve duration of the intake valves is not promptly eliminated.